The present invention relates to an excitation control apparatus for a synchronous machine, which is synchronized or parallel-connected with an electric power line within a short period of time.
In general, when a synchronous machine is synchronized with an electric power line, they must be synchronized with each other after differences between respective voltages, frequencies, and phases are minimized to an appropriately small value.
In particular, conventional large synchronous machines require a relatively long period of time to adjust their frequency and phase. However, since they are continuously operated for a long period of time, such a long adjustment time is not regarded as a problem.
However, recently, large machines tend to be frequently driven and stopped in accordance with variations in loads of the electric power line because of the improvement in the speed governing device in the synchronous machine or in order to improve the operation efficiency of the synchronous machine. Therefore, a time required for controlling the voltage is not negligible.
When a synchronous motor is started by low-frequency starting equipment, conditions for a voltage must be quickly satisfied, and as soon as conditions for speeds and phases are satisfied, they must be synchronized. Therefore, a demand has arisen for decreasing the time required for controlling the voltage of the synchronous machine to be as short as possible.
FIG. 1 is a block diagram of a synchronous machine using a conventional thyristor exciter.
Field winding 2 of synchronous machine 1 is connected to thyristor exciter 4 through field circuit breaker 3. The output from synchronous machine 1 is supplied, as an AC power source, to thyristor exciter 4 through field transformer 5. Initial excitation power source 7 is connected in parallel with thyristor exciter 4 through initial excitation switch 6.
Automatic voltage adjusting apparatus 8 comprises voltage detector 10, which detects the output voltage from synchronous machine 1 through first voltage transformer 9. The detection value of detector 10 and the set value of voltage setter 11 are input to subtracter 12 to obtain a difference therebetween. The difference is then supplied to gate pulse generator 13. Gate pulse generator 13 generates pulses which are phase-controlled in accordance with the difference, and supplies them to thyristor exciter 4 through pulse transformer 14, thereby controlling the output voltage from synchronous machine 1 to correspond with the set value of voltage setter 11.
Synchronous machine 1 is synchronized with electric power line 16 through synchronous circuit breaker 15. Synchronism indicator 18 and voltage control apparatus 19 are installed to have as their inputs the secondary sides of first voltage transformer 9 and second voltage transformer 17 connected to line 16.
Voltage control apparatus 19 detects the voltage from synchronous machine 1 through first voltage transformer 9 with synchronous machine voltage detector 20, and detects the voltage from line 16 through second voltage transformer 17 with line voltage detector 21. These detectors supply their detection values to subtracter 22 to detect a difference therebetween. Thereafter, voltage controller 23 controls the set value of voltage setter 11 of apparatus 8 so that the difference is decreased to zero.
FIG. 2 is a graph showing the relationship between a change in the voltage of synchronous machine 1 over time and the voltage of line 16 from when a voltage is generated from a non-excited state until machine 1 is synchronized with line 16 in the case wherein machine 1 is a power generator.
Note that the abscissa indicates time, and the ordinate indicates voltage Vl of line 16 or voltage Vg of machine 1. Reference symbol Vg0 denotes the voltage of machine 1 corresponding to an initial set value of voltage setter 11.
When circuit breaker 3 and switch 6 are closed at time t0, the voltage of DC power source 7 is applied to field winding 2, and a field current gradually increases in accordance with the time constant of winding 2.
At time t1, when output voltage Vg of machine 1 is increased to a value that causes gate pulse generator 13 of apparatus 8 to generate gate pulses, thyristor exciter 4 is controlled by apparatus 8. The current from exciter 4 is supplied to winding 2 through transformer 5 using the output from machine 1 as a power source.
When exciter 4 is operated, switch 6 is opened, and output voltage Vg of machine 1 immediately increases upon operation of apparatus 8. At time t2, voltage Vg reaches voltage Vg0 corresponding to the set value of setter 11.
When the voltage of machine 1 is stabilized, voltage control then starts.
When the voltage condition, detected by indicator 18 for indicating the difference between voltages Vg of machine 1 and voltage Vl of line 16, is smaller than constant value V, circuit breaker 15 is closed to synchronize machine 1 with line 16.
When the difference is smaller than value .DELTA.V and the voltage of machine 1 is stabilized at voltage Vg to correspond to the initial set value of setter 11, the synchronous voltage condition is satisfied.
However, as shown in FIG. 2, voltage Vl of line 16 is not always constant, and is often larger than constant value .DELTA.V. In this case, apparatus 19 is operated so that voltage setter 11 is driven to decrease the difference between the detection values from detectors 20 and 21. Thus, voltage Vg of machine 11 is approximated to voltage Vl of line 16, and the difference becomes smaller than constant value .DELTA.V at time t3. Therefore, when difference .DELTA.V between voltage Vg of machine 1 and voltage Vl of line 16 is small, a time interval between times t2 and t3 is relatively short. However, when difference .DELTA.V is large, this time interval is long.